There is known an automatic transmission of a type incorporating a plurality of frictional coupling devices which are disposed at respective different radial positions so as to reduce the required axial dimension of the automatic transmission. JP-7-198037 A discloses an example of such type of automatic transmission in which the frictional coupling devices are disposed at the respective radial positions so as to improve the efficiency of lubrication of the frictional coupling devices.
In the automatic transmission disclosed in the above-identification publication, a plurality of drums in which pistons of the respective frictional coupling devices are axially slidably received are fixed to each other by welding. However, a process of welding together the plurality of drums requires a large number of process steps, which require a high cost of manufacture of the automatic transmission. Thus, the known automatic transmission is not designed for economical manufacture.